1. Field of the Invention
The present invention relates to an optical data recording medium such as, for example, an optical disc or the like and a manufacturing method thereof, and more specifically, to a means for preventing moisture absorption and swelling of a resin layer formed on one side of a transparent substrate.
2. Prior Art
Conventionally, there has been known an optical disc of an MCAV (modified constant angular velocity) rotation drive system or MCLV (modified constant linear velocity) rotation drive system which has a recording capacity larger than that of an optical disc of a CAV (constant angular velocity) rotation drive system and can access at a speed higher than that of an optical disc of a CLV (constant linear velocity) rotation drive system.
FIGS. 18 and 19 shows an example of the conventionally proposed optical disc of the MCAV or MCLV system.
FIG. 18 is a plan view of this optical disc, wherein a ring-shaped area 3 of a disc-shaped substrate 2 having a center hole 1 except the innermost and outermost circumferences thereof serves as a recording area. The recording area 3 is divided into a plurality of blocks 4a, 4b, 4c . . . 4x each area having a different radial and further each of the blocks 4a, 4b, 4c . . . 4x is divided in the circumferential direction into a plurality of sectors 5a, 5b, 5c . . . 5x. Each of the sectors 5a, 5b, 5c, . . . 5x has the substantially same length and the number of the sectors of the outermost circumferential block 4x is designed to be about twice the number of the sectors of the intermost circumferential block 4a.
The recording area 3 has microminiature guide grooves of a line width of about 0.3 .mu.m-0.8 .mu.m having a constant track pitch defined concentrically or spirally from the innermost circumferential portion to the outermost circumferential portion thereof (not shown in FIG. 18) to enable recording/reproducing light to be tracked. A prepit is formed on the head portion of each of the sectors 5a, 5b, 5c . . . 5x on the guide grooves or between two guide grooves adjacent each other to indicate the address or the like of the sector. The group of the prepits seems to be disposed linearly along the boundary of each separated sector, as shown by numeral 6, when the optical disc is obtained from the flat surface direction thereof.
As described above, since the optical disc of the MCAV system or MCLV system has the recording area divided into a plurality of the blocks 4a, 4b, 4c . . . and further the respective blocks are divided into the different sectors, this optical disc does not have prepits disposed linearly from the innermost circumferential portion to the outermost circumferential portion of the recording areas like the optical disc of the CAV system, and thus, as shown in FIG. 18, there is formed an area 7 where a prepit is disposed next to a guide groove in the boundary portion of each of the blocks 4a, 4b, 4c . . . 4x.
FIG. 19 is a cross sectional view of a main part of this optical disc, wherein a photo-curing resin layer 10 is formed on one side of a substrate 2 composed of a flat and disc-shaped glass plate, prepits 8 and guide grooves (pregrooves) 9 are transferred on the photo-curing resin layer 10 and at least a thin film layer 11 including a recording film or a reflecting film is formed on the photo-curing resin layer 10. The prepit 8 is formed to a trapezoidal groove shape having a depth of .lambda./4 n, where .lambda. represents a wavelength of recording/reproducing light and n represents a refraction factor of the photo-curing resin layer 10 and the guide groove 9 is formed to a V-shaped having a depth of about .lambda./6 n-.lambda./8 n, where .lambda. represents a wavelength of recording/reproducing light and n represents a refraction factor of the photo-curing resin layer 10.
In the MCAV system, a recording capacity is increased in such a manner that the above optical disk is rotated at a constant angular velocity and the closer to the outer circumference the blocks are located, the more the length of a writing signal to a data signal having the same signal length is reduced.
On the other hand, in the MCLV system, a recording capacity is increased in such a manner that the length of a writing signal to a data signal having the same signal length is kept constant regardless the blocks are located closer to the inner circumference or the outer circumference and the closer to the outer circumference the blocks are located the more the rpm of the disc is reduced.
In general, the thin film layer 11 including the recording film is formed by a sputtering method or a vacuum vapor deposition method because it can efficiently form a thin film and has the high uniformity of the thin film. Since, however, sputtered particles from a target or a vaporized material from a vaporizing source are less penetrated, the sputtered particles or the vaporized material is difficult to be deposited on a side wall 8a of the prepit 8 and thus this portion has a thickness which is much thinner than other portions, as shown in FIG. 19.
When the thin film layer 11 is water-impermeable, water is prevented from permeating through the substrate 2 and the portion where the thin film layer 11 is formed to a desired thickness. Since, however, the photocuring resin used to a conventional optical disk has a high water absorption of 2-3% and thus when the optical disc is left in air, the water in the air permeates into the photo-curing resin layer 10 through the side wall 8a of the prepit 8 where the thin film layer 11 has a very thin thickness, as shown by the arrows A in FIG. 20, so that the photo-curing resin 10 is locally swelled.
Accordingly, in an optical disc in which the prepit 8 is disposed next to the guide groove 9 of an adjacent track like the above optical disc of the MCAV or MCLV system, the guide grooves 9 of a plurality of recording tracks passing through the vicinity of an area adjacent to this prepit 8 is inclined in a radial direction, as shown in FIG. 20.
When this phenomenon arises, the intensity of the light reflected from or passing through the respective surfaces of the V-shaped guide groove 9 is unbalanced to cause track offsets 12, as shown in FIG. 21, so that a tracking noise is produced. As a result, a recording portion (a pit, magnetized domain or the like) is formed in such a manner that it is biased to an adjacent track side so that there is a drawback that a cross-talk is produced and a signal cannot be recorded or reproduced in the worst case.
Although the above description is made with reference to an example such as the optical disc of the MCAV or the MCLV system, the same problems also arise in an optical disc of a so-called CLV system (constant linear velocity system) in which guide grooves are formed concentrically or spirally and prepits are formed along these guide grooves at equal intervals. Further, in an optical disc having an ablation type recording pit, since an area where a recording pit is disposed next to a guide groove of an adjacent track is produced after recording, a drawback similar to that mentioned above may be caused. Furthermore, in a card-shaped optical data recording medium, the same drawback may arise if a preformat is such that there is an area where a guide groove is disposed next to a prepit.